ath9k_hw: Fix and complete force bias for AR5416

Force bias is a fix for usage of AR5416 radios on the 2.4 GHz band
for orientation sensitivity. This was only partially implemented
with the ath9k_hw_decrease_chain_power() but first -- this was being
called for all chipsets which is not correct and second -- it was
missing the actual orientation code.

We now ensure to only enable force bias only for AR5416 and BUG_ON()
on other chipsets. Although ath9k_hw_decrease_chain_power() was enabled
for newer chipsets I suspect that it never ran unless the EEPROM had
ATH9K_ANT_FIXED_A or ATH9K_ANT_FIXED_B for antenna diversity.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>